Partable laminate and method for parting long-term structural bonds

The present disclosure relates to a laminate designed and equipped to be separated after long-term bonding, comprising a first layer of adhesive, a separation layer, and a second layer of adhesive. The present disclosure further encompasses a method for parting a long-term structural bond produced by means of such a laminate.

In repair shops and in the end-of-life recycling of electronic devices, the desire to be able to repair electronic devices or else automobiles, or to be able as extensively as possible to disassemble and/or recycle them, is gaining in importance for not just environmental reasons but also economic reasons.

There are different kinds of electronic devices here, differing in their recyclability and also in the degree of recycling:

Electrical and electronic devices in particular contain a multiplicity of substances and materials. If used electrical and electronic devices are disposed of improperly, such as via the household garbage, for example, environmental risks may arise from the pollutants they still contain in some cases. As well as pollutants such as heavy metals and HCFCs, however, used electrical and electronic devices also contain a range of valuable substances, which should be recovered and therefore recirculated. Where, conversely, used electrical and electronic devices are disposed of properly, it is possible to replace primary raw materials (and hence their costly and laborious extraction) and to make a substantial contribution to the preservation of the natural resources.

In order to be able to achieve these objectives, there are specific obligations imposed on all relevant actors (manufacturers, trade, municipalities, owners, waste managers) in Germany by the law governing the sale, return, and environmentally sound disposal of electrical and electronic equipment (Electrical and Electronic Equipment Law—ElektroG) in implementation of Directive 2012/19/EU concerning waste electrical and electronic equipment (WEEE). By avoiding waste, through reasonable tests for possibilities of preparation for the re-use of entire devices or individual components, and by requirements regarding the more extensive recovery of value from wastes, the aim is to achieve a substantial contribution to preserving natural resources and to reducing pollutant emissions.

Corresponding recycling-friendly designs are needed which enable on-demand disassembly (“debonding on demand”). The recycling-friendly designs include repartable adhesive bonds.

The reason is that, in small electronic devices in particular, there is a very sharply increasing trend toward adhesively bonding parts, usually on a long-term basis, rather than connecting them in a way which can be undone mechanically.

Film laminates in the form of double-sided adhesive tapes are employed, for example, for bonding two components to one another. In general the intention is for these components to be bonded to one another on a long-term basis by such a film laminate. This is intended to result in a correspondingly long life and durability of the bond and/or the product. Examples of components joined to one another in this way are touch panels of the kind employed in computer screens or mobile electronic devices. If one of the two components is damaged, it is completely impossible, or possibly only on application of substantial resource (force), to separate the bonded assembly again in order to replace a component. There is also the risk of the component that is not damaged suffering damage in the course of the separation.

DE 10 2020 209 557 A1 discloses a film laminate designed and equipped to be separated after long-term bonding, comprising the following layers:

In this case a metal is removed by laser, leading to the separation.

Translucency is the partial light transmissiveness of a body. The word derives from the Latin lux for light. Wax, the human skin, leaves, and many other substances are translucent, since they transmit light partially, but are not transparent. In delimitation from transparency, translucency may be described as light transmissiveness. The reciprocal property to translucency is opacity. Hence where a substance possesses high translucency, it has low opacity, and vice versa.

Light transmissiveness in the sense of the disclosure means transmissive at the respective wavelength of the light. This means that, for example, a black body (for example, a black-colored polymer) is opaque in the range of light that is visible for humans, but is translucent in the nonvisible range such as NIR, meaning that radiation in this wavelength range is able to pass through it.

EP 3 390 553 A1 relates to a method for bonding two surfaces by means of a reactive adhesive film system comprising at least two adhesive films (F1 and F2), the adhesive films each comprising at least one reactive component (R1 and R2), the bonding being brought about by a reaction which requires the presence of both reactive components (R1 and R2), where prior to the bonding there is a parting layer (T) which is impervious for the reactive components (R1 and R2) between the adhesive films (F1 and F2) that are to be brought into contact with one another with the reaction. In order to produce the bond, the parting layer (T) is removed over at least part of its area by means of a laser, and so the adhesive films (F1 and F2) come into direct contact with one another and the reaction ensues in the presence of the two reactive components (R1 and R2).

The parting layer may be a metal layer. This may be a metal foil which is introduced between the adhesive films during the production of the adhesive tape, by means of a laminating operation, for example.

DE 81 30 861 U1 discloses laser-writable labels having an outer coating material layer and, disposed below it, a second coating material layer, with the coating material layer being produced from polyurethane acrylate and hexanediol bisacrylate. Building on this, DE 100 48 665 A1 discloses laser-writable labels having an electron beam-cured coating material layer. A method for producing such laser-writable labels is described in DE 101 42 638 A1, wherein an engraving layer with a UV-curable coating material is incorporated. By means of an additional compensation layer, DE 10 2005 061 125 A1 produces labels which attenuate deterioration due to high temperatures above 140° C.

The use of lasers for ablation is widespread—for example, in micromachining, certain laser beam sources can be used for ablative operations. Extremely thin layers can be removed from substrates, since the local heating leads to a particulate debris or to carbonization/evaporation. In order to realize ablation operations as sparingly as possible, lasers in the wavelength range from 800 to 2000 nm are primarily employed. For photochemical reactions with low exposure to heat, excimer lasers are frequently used. Excimer laser means that the laser beams are situated in the UV wavelength range.

The table below lists the typical properties of an Nd:YAG laser.

Furthermore, USP (ultra-short pulse) lasers have proven particularly suitable.

Ultra-short pulse lasers are laser beam sources which emit pulsed laser light having pulse durations in the range of picoseconds and femtoseconds.

Ultra-short pulse lasers emit pulses of light in which the light energy is compressed to extremely short times, with luminous powers in the megawatt range being achieved during the pulse. By means of appropriate spatial focusing, it is therefore possible to obtain intensities of many gigawatts per square centimeter. At such high intensities, there are nonlinear effects in the interaction between light and matter. One of these effects is that known as multiphoton absorption, which results in virtually any material being ablatable at sufficiently high intensities. This is true particularly of femtosecond lasers. In this case no part is played by their absorption, their hardness or their vaporization temperature, and even challenging materials such as composite materials can be readily machined.

A further advantage of ultra-short pulse lasers is their high precision. Focal diameters in the micrometer range and the low energy input per pulse enable laser ablation with high spatial resolution. The rule here is as follows: the shorter the pulse duration, the less the extent to which the surrounding material is damaged by the laser beam and the more precise the degree of metering with which the material can be ablated. The results are clean cut edges without burring, and so there is no need for reworking. In metal working, nanosecond pulses are usually sufficient; more elaborate machining requires picosecond pulses, while, for nonmetallic materials, such as ceramics, polymers, and many composite materials, femtosecond pulses are employed. The lower level of ablation of material accompanying the shorter pulse duration, however, means that machining takes longer overall. One objective in the current development work on ultra-short pulse lasers, therefore, is to increase the pulse repetition rates (number of laser pulses per second). This will raise the average power and hence the throughput in manufacturing. In the laboratory, femtosecond lasers with an average power of more than 1 kilowatt have already been demonstrated. They have pulse repetition rates of 20 megahertz, pulse energies of 55 microjoules and pulse durations of 600 femtoseconds. Available commercially today there are femtosecond lasers having average powers of not more than a few hundred watts, operating in general with ytterbium-doped laser crystals.

It is an object of the present disclosure, therefore, to provide a laminate which on the one hand enables long-term and reliable bonding of two components with one another, but on the other hand, as and when required, enables clean and reliable separation of the components.

The object is achieved in accordance with the disclosure by means of a film laminate as described in claim. Advantageous embodiments are reproduced in the dependent claims. Also part of the disclosure are a method for parting a long-term structural bond produced by means of a laminate of the disclosure, by removing at least part of the area of the separation layer by means of laser irradiation and separating the laminate into a first part-laminate and a second part-laminate, and also proposed uses of the laminate of the disclosure.

The present disclosure relates accordingly to a laminate designed and equipped to be separated after long-term structural bonding, comprising the following layers:

In accordance with the disclosure the separation layer is characterized by the following properties:

The first adhesive layer and/or the second adhesive layer are laser beam-translucent. Furthermore, the first adhesive layer or the second adhesive layer, or the first adhesive layer and the second adhesive layer, comprise a reactive or latent-reactive adhesive or consist of a reactive or latent-reactive adhesive.

With a laminate of this kind it is possible for two substrates—for example, glass/glass, glass/metal, glass/plastic, or plastic/plastic—to be permanently bonded. As a result of the controlled removal of the thin separation layer, the composite adhesion between the two layers of adhesive can be reduced to an extent that enables very easy separation of the layers—in the best case, the composite adhesion is eliminated almost entirely. This makes it possible to achieve what is referred to as reworkability, meaning that an adhesive bond which has actually been made as a connection that can no longer be altered can nevertheless be undone again. This removal of the separation layer is accomplished by ablation.

The present disclosure further relates to a method for parting a long-term structural bond produced by means of a laminate of the disclosure, wherein at least part of the area of the separation layer is removed by means of laser irradiation and the laminate is separated into a first part-laminate and a second part-laminate.

This preferably involves the application, to at least one of the part-laminates, of forces which increase the spacing of the two part-laminates from one another. Accordingly, the laminate can be separated into two part-laminates in a particularly effective and reliable way.

A typical construction of a laminate of the disclosure therefore looks as follows:

It is important that either the first layer of adhesive is translucent for the laser radiation used, and/or the second layer of adhesive, so that the laser is able to reach the separation layer. The same applies to the substrate for bonding, at least on the side from which the laser radiation is introduced. This substrate as well must be transmissive for the laser radiation. The separation layer itself absorbs the laser radiation.

The separation layer consists of a cured coating material, preferably a radiation-cured coating material, more particularly of an electron beam-cured or UV-cured coating material.

In accordance with the disclosure, the coating material is admixed with laser absorbers, i.e., laser-absorbing pigments, in order to achieve an extremely efficient take-up of energy during the lasering treatment. These laser-sensitive pigments at the same time produce a coloring of the separation layer. Titanium dioxide and/or carbon black therefore serve as typical laser absorbers which at the same time bring about a coloration.

If a laser-sensitive pigment is present in the separation layer, through the addition of titanium dioxide and/or carbon black, for example, it is also possible, moreover, for other coloring pigments to be added, and so it is possible to produce a coating material layer of any desired color. The actual coloring pigment of the coating material in that case no longer needs to fulfill any particular absorption properties in respect of the laser absorption.

Suitable separation layers comprise radiation-curable systems such as unsaturated polyesters, epoxy acrylates, polyester acrylates, and urethane acrylates, of the kind also used for UV printing inks, and more particularly those composed of base polymers according to DE U 81 30 816, namely aliphatic urethane acrylate oligomers.

In principle for the separation layer of the disclosure it is possible to use in particular four types of coating material—for example, acid-curing alkyd-melamine resins, addition-crosslinking polyurethanes, radically curing styrene coating materials, and similar. Particularly advantageous, however, are radiation-curing coating materials, since they cure very rapidly without laborious evaporation of solvents or exposure to heat. Such coating materials have been described, for example, by A. Vrancken (Farbe and Lack 83, 3 (1977) 171).

According to one preferred embodiment, the separation layer consists of a single coating material layer, which in particular is electron beam-cured.

For this purpose the coating material layer preferably employed is applied to a liner and cured by exposure to an electron beam of high energy (150 to 500 kV) under effectively oxygen-free conditions.

With particular advantage the coating material comprises a cured acrylate coating composition. The cured acrylate coating composition is based, according to one particularly advantageous embodiment, on a composition comprising

In one preferred embodiment of the present disclosure, the composition on which the acrylate coating composition is based comprises 50 to 60 wt %, preferably 52 to 58 wt %, of the trifunctional oligomer A, 5 to 15 wt %, preferably 8 to 12 wt %, of the trifunctional monomer B, and 5 to 15 wt %, preferably 8 to 12 wt %, of the difunctional monomer C.

The amount of the laser-sensitive pigment within the acrylate coating compositions of preferred embodiments is dependent on the nature of the pigment used.

In principle the laser-sensitive pigments are admixed in an order of magnitude of 1 wt % up to not more than 40 wt %, preferably in amounts of 2 to 28 wt % or in amounts of 5 to 15 wt %, based on the total weight of the coating material layer.

In the case of carbon black as coloring pigment (in order to achieve the preferred black coloration), for example, 2 to 7 wt % are preferred, whereas in the case of TiO, for whitening, preferably 15 to 40 wt %, more preferably 22 to 28 wt %, are used. Preference is given to using titanium dioxide in the rutile modification (“TiO”, examples being rutile grades from Kronos).

The trifunctional oligomer A, the trifunctional monomer B, and the difunctional monomer C are also referred to below as component A, component B, and component C, respectively.

Compositions comprising components A, B, and C and also the coloring pigment in the stated amount produce particularly temperature-resistant cured acrylate coating compositions.

The separation layer may be provided by curing a composition comprising the components A, B, and C and also the laser-sensitive pigment. For this purpose the composition is crosslinked by means of IR or UV radiation or electron beam curing (hereinafter EBC). Crosslinking by means of EBC is preferred.

The trifunctional oligomer A is an oligomer having three unsaturated (meth)acrylate units per molecule, with a number-average molecular weight M(determined by gel permeation chromatography (GPC)) of preferably between 1000 and 5000 g/mol, preferably between 1400 and 3600 g/mol, preferably between 1800 and 2200 g/mol, more preferably between 1900 and 2100 g/mol. Where the molecular weight Mis within the stated range, this has a positive influence on the long-term temperature resistance of the cured acrylate coating composition, allowing particularly dimensionally stable contrast layers to be obtained.

In one preferred embodiment, the trifunctional oligomer A is selected from the group of polyurethane tri(meth)acrylates and polyester tri(meth)acrylates, of which polyurethane tri(meth)acrylates are particularly preferred. The expression “(meth)acrylate” encompasses acrylates, methacrylates, and mixtures thereof. The trifunctional oligomer A is preferably a polyurethane tri(meth)acrylate, more preferably a polyurethane triacrylate. Polyurethane tri(meth)acrylates are oligomers having in each case three unsaturated (meth)acrylate groups per molecule and also a plurality of, in other words at least two, urethane units. Examples of preferred polyurethane triacrylates are the aliphatic urethane triacrylates CN9260D75® and CN9278D80® from Sartomer, of which CN9260D75® is particularly preferred.

The trifunctional monomer B contains three unsaturated (meth)acrylate units per molecule and in one preferred embodiment of the disclosure has a molecular weight of 300 to 1000 g/mol, preferably 350 to 800 g/mol, preferably 350 to 600 g/mol, more preferably 400 to 450 g/mol. Component B is preferably selected from the group consisting of propoxylated and ethoxylated glycerol tri(meth)acrylates and propoxylated and ethoxylated trimethylolpropane tri(meth)acrylates of the general Formula (I) or mixtures thereof: